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Face detection
Many slides adapted from P. Viola
• Basic idea: slide a window across image and evaluate a face model at every location
Face detection
Challenges of face detection• Sliding window detector must evaluate tens of
thousands of location/scale combinations• Faces are rare: 0–10 per image
• For computational efficiency, we should try to spend as little time as possible on the non-face windows
• A megapixel image has ~106 pixels and a comparable number of candidate face locations
• To avoid having a false positive in every image image, our false positive rate has to be less than 10-6
The Viola/Jones Face Detector• A seminal approach to real-time object
detection • Training is slow, but detection is very fast• Key ideas
• Integral images for fast feature evaluation• Boosting for feature selection• Attentional cascade for fast rejection of non-face windows
P. Viola and M. Jones. Rapid object detection using a boosted cascade of simple features. CVPR 2001.
P. Viola and M. Jones. Robust real-time face detection. IJCV 57(2), 2004.
Image Features
“Rectangle filters”
Value =
∑ (pixels in white area) –∑ (pixels in black area)
Example
Source
Result
Fast computation with integral images• The integral image
computes a value at each pixel (x,y) that is the sum of the pixel values above and to the left of (x,y), inclusive
• This can quickly be computed in one pass through the image
(x,y)
Computing the integral image
Computing the integral image
Cumulative row sum: s(x, y) = s(x–1, y) + i(x, y) Integral image: ii(x, y) = ii(x, y−1) + s(x, y)
ii(x, y-1)
s(x-1, y)
i(x, y)
MATLAB: ii = cumsum(cumsum(double(i)), 2);
Computing sum within a rectangle
• Let A,B,C,D be the values of the integral image at the corners of a rectangle
• Then the sum of original image values within the rectangle can be computed as:
sum = A – B – C + D• Only 3 additions are
required for any size of rectangle!
D B
C A
Example
-1 +1+2-1
-2+1
Integral Image
Feature selection• For a 24x24 detection region, the number of
possible rectangle features is ~160,000!
Feature selection• For a 24x24 detection region, the number of
possible rectangle features is ~160,000! • At test time, it is impractical to evaluate the
entire feature set • Can we create a good classifier using just a
small subset of all possible features?• How to select such a subset?
Boosting• Boosting is a classification scheme that works
by combining weak learners into a more accurate ensemble classifier• A weak learner need only do better than chance
• Training consists of multiple boosting rounds• During each boosting round, we select a weak learner that
does well on examples that were hard for the previous weak learners
• “Hardness” is captured by weights attached to training examples
Y. Freund and R. Schapire, A short introduction to boosting, Journal of Japanese Society for Artificial Intelligence, 14(5):771-780, September, 1999.
Training procedure• Initially, weight each training example equally• In each boosting round:
• Find the weak learner that achieves the lowest weightedtraining error
• Raise the weights of training examples misclassified by current weak learner
• Compute final classifier as linear combination of all weak learners (weight of each learner is directly proportional to its accuracy)
• Exact formulas for re-weighting and combining weak learners depend on the particular boosting scheme (e.g., AdaBoost)
Y. Freund and R. Schapire, A short introduction to boosting, Journal of Japanese Society for Artificial Intelligence, 14(5):771-780, September, 1999.
Boosting illustration
Weak Classifier 1
Boosting illustration
WeightsIncreased
Boosting illustration
Weak Classifier 2
Boosting illustration
WeightsIncreased
Boosting illustration
Weak Classifier 3
Boosting illustration
Final classifier is a combination of weak classifiers
Boosting vs. SVM• Advantages of boosting
• Integrates classification with feature selection• Complexity of training is linear instead of quadratic in the
number of training examples• Flexibility in the choice of weak learners, boosting scheme• Testing is fast• Easy to implement
• Disadvantages• Needs many training examples• Often doesn’t work as well as SVM (especially for many-
class problems)
Boosting for face detection
• Define weak learners based on rectangle features
⎩⎨⎧ >
=otherwise 0
)( if 1)( tttt
t
pxfpxh
θ
window
value of rectangle feature
parity threshold
• Define weak learners based on rectangle features
• For each round of boosting:• Evaluate each rectangle filter on each example• Select best threshold for each filter • Select best filter/threshold combination• Reweight examples
• Computational complexity of learning: O(MNK)• M rounds, N examples, K features
Boosting for face detection
Boosting for face detection• First two features selected by boosting:
This feature combination can yield 100% detection rate and 50% false positive rate
Boosting for face detection• A 200-feature classifier can yield 95% detection
rate and a false positive rate of 1 in 14084
Not good enough!
Receiver operating characteristic (ROC) curve
Attentional cascade
• We start with simple classifiers which reject many of the negative sub-windows while detecting almost all positive sub-windows
• Positive response from the first classifier triggers the evaluation of a second (more complex) classifier, and so on
• A negative outcome at any point leads to the immediate rejection of the sub-window
FACEIMAGESUB-WINDOW
Classifier 1T
Classifier 3T
F
NON-FACE
TClassifier 2
T
F
NON-FACE
F
NON-FACE
Attentional cascade
• Chain classifiers that are progressively more complex and have lower false positive rates: vs false neg determined by
% False Pos
% D
etec
tion
0 50
0
100
FACEIMAGESUB-WINDOW
Classifier 1T
Classifier 3T
F
NON-FACE
TClassifier 2
T
F
NON-FACE
F
NON-FACE
Receiver operating characteristic
Attentional cascade• The detection rate and the false positive rate of
the cascade are found by multiplying the respective rates of the individual stages
• A detection rate of 0.9 and a false positive rate on the order of 10-6 can be achieved by a 10-stage cascade if each stage has a detection rate of 0.99 (0.9910 ≈ 0.9) and a false positive rate of about 0.30 (0.310 ≈ 6×10-6)
FACEIMAGESUB-WINDOW
Classifier 1T
Classifier 3T
F
NON-FACE
TClassifier 2
T
F
NON-FACE
F
NON-FACE
Training the cascade• Set target detection and false positive rates for
each stage• Keep adding features to the current stage until
its target rates have been met • Need to lower AdaBoost threshold to maximize detection (as
opposed to minimizing total classification error)• Test on a validation set
• If the overall false positive rate is not low enough, then add another stage
• Use false positives from current stage as the negative training examples for the next stage
The implemented system
• Training Data• 5000 faces
– All frontal, rescaled to 24x24 pixels
• 300 million non-faces– 9500 non-face images
• Faces are normalized– Scale, translation
• Many variations• Across individuals• Illumination• Pose
System performance• Training time: “weeks” on 466 MHz Sun
workstation• 38 layers, total of 6061 features• Average of 10 features evaluated per window
on test set• “On a 700 Mhz Pentium III processor, the
face detector can process a 384 by 288 pixel image in about .067 seconds” • 15 Hz• 15 times faster than previous detector of comparable
accuracy (Rowley et al., 1998)
Output of Face Detector on Test Images
Other detection tasks
Facial Feature Localization
Male vs. female
Profile Detection
Profile Detection
Profile Features
Summary: Viola/Jones detector• Rectangle features• Integral images for fast computation• Boosting for feature selection• Attentional cascade for fast rejection of
negative windows